Core material

ABSTRACT

The present invention relates to a drapable core material suitable for use in closed mould systems, hand lay-up applications and/or spray-up applications, preferably having a compression-resistance of more than 30% at 1 bar pressure, wherein core material is based on at least one fibrous web containing a foam-structure within the web, said foam-structure being formed of a plurality of members, which members are separated from each other by channels which channels are permeable to resin. The invention further relates to a laminate comprising such core material, a method for manufacturing the core material and a method for manufacturing such laminate.

The invention relates to a core material for use in the production offibre reinforced plastics materials, more in particular suitable forapplication in closed mould systems, spray up applications and/orhand-lay up applications.

Plastics reinforced with fibrous webs, are often used for manufacturingshaped articles such as automotive or industrial parts, e.g. tanks, bathtubs, road signs, cladding panels, boats, caravans, etc.

Fibrous webs are suitable as a reinforcement for all kinds of curedsynthetic plastics materials, such as polyester resin or epoxy resin.Generally, the incorporation of a fibrous web in a resin materialresults in increased strength, stiffness, fatigue life, fracturetoughness, environmental resistance, increased temperature stability,reduced weight and reduced manufacturing cost of said resin material.

Use of core materials in fibre reinforced plastics has already beenknown for decades. The aim thereof is on the one hand to decrease theamount of resin required, resulting in cost and weight savings, and onthe other hand to improve some mechanical properties of the material,more in particular the bending stiffness.

U.S. Pat. No. 3,676,288 discloses non-expanded micro-spheres which areapplied to or incorporated in a fibrous web by means of a binder, forexample, a polyacrylonitrile latex. As the binder is dried andcross-linked, the spheres are attached to the fibrous web and expanded.

EP-A-0 190 788 is directed to the use of a fibrous web, incorporatingmicro-spheres, for the manufacture of objects reinforced with such afibrous web. According to said patent application, the micro-spheres aremainly contained within the web and arranged in a pattern in which areasof the web, which contain micro-spheres, are separated from each otherby areas which contain virtually no micro-spheres.

In the production of fibre reinforced plastics materials two prominentmethods are available, one being based on the manual impregnation of thefibre materials (hand lay-up; spray-up) and another being based on theuse of closed moulds. In the latter system—which is usuallyautomated—the fibre reinforcing material is placed in a mould, which isclosed and subsequently filled with resin. An important advantage ofthese closed mould systems resides inter alia in the reproducibility ofthe properties of the product (better tolerances), in environmentalconsiderations, in enhanced surface properties and in enhancedmechanical properties. It is also possible to apply higher fibre volumefractions. Use of the above described core materials in closed mouldsystems have for a long time given rise to difficulties in reconcilingthe various requirements to be met by a core material for use therein.These properties are inter alia.

-   -   good compression resistance,    -   fast flow of resin through the core material in all directions,    -   low resin uptake,    -   decreased shrink (i.e. compensate for resin shrinkage), and    -   good drapability (i.e. low bending stiffness).

In particular the first two requirements have been very difficult toreconcile for a long time. It will be clear, that the open structurethat is necessary for obtaining a good resin flow in the plane of thecore material, will tend to be at the expense of thecompression-resistance. Further, a low resin uptake, which can beobtained by a large volume of foam in the web, will be incompatible withthe good flow of resin. Also the drapability characteristics are noteasily compatible with the compression resistance and the low resinuptake. In order to address these requirements a core material has beendeveloped as is disclosed in EP 1 010 793. In a preferred embodiment,the core material contains micro-spheres distributed over the corematerial in a regular pattern.

It has however been found that the surface quality of these prior artmaterials, such as the material disclosed in EP 1 010 793 is not alwayssatisfactory, in particular not with respect to the visual appreciationof the surface (in particular with respect to the occurrence of aprint-through effect) or surface finish. Such visual appreciation orsurface finish can for example be important in shaped articles such aspanels for cars, trucks, etc. Methods to assess the visual appreciationinclude panel tests.

It is also possible to assess a quantitative indication of the surfacefinish by diffraction measurement of the surface. The orange peel valueof the surface is for example a suitable parameter to this purpose.Instruments to measure the orange peel value are known in the art andare commercially available, e.g. instruments using the D-Sighttechnique, developed by Diffracto Ltd (Canada). This technique isdiscussed in Reynolds et al. “Theory and applications of a surfaceinspection technique using double-pass retroreflection”, OpticalEngineering, Vol 32, No 9, pp. 2122-2129, 1993 and in J. H. Heida and A.J. A. Bruinsma; “D-Sight Technique for Rapid Impact Damage Detection onComposite Aircraft Structures”; Presented at: the 7th EuropeanConference on Non-Destructive Testing in Copenhagen, 26-29 May 1998;available at NDT.net—June 1999, Vol. 4 No. 6, U.S. Pat. No. 4,863,268,U.S. Pat. No. 5,075,661.

Accordingly, it is an object of the present invention to provide a newcore material, which can be used as an alternative to known corematerials, in particular for the manufacture of shaped objects whereinthe visual appreciation of the surface is relevant. More in particularit is an object to provide such a core material wherein the material issuitable for use in closed mould systems, spray up applications, and/orhand lay-up applications.

In a particular aspect, an object of the invention is to provide a corematerial which may be used in the manufacture of a shapedarticle—typically involving the impregnation of the core material with aresin—with an improved orange peel.

More in particular, it is an object to provide a core material which issuitable to be used in the manufacture of a shaped article to obtain ashaped article having an orange peel value as defined herein of lessthan 30.

It has been found that one or more of these objects can be realised by acore material—which is in general drapable and preferably has a highcompression resistance—which core material contains members ofrelatively dense material and channels, being open or containingmaterial with a relatively low density, wherein, the members andchannels are present in a specific pattern. Namely, in accordance withthe invention, the members and channels are irregularly distributedand/or the members and channels have specific dimension, namely havingrelatively small members and relatively small channels.

Accordingly, the present invention relates to a core material, inparticular a core material suitable for use in closed mould systems,spray-up applications and/or hand-lay up applications, said corematerial in general being drapable—and preferably having acompression-resistance of more than 30% at 1 bar pressure—wherein thecore material is based on at least one fibrous web containing afoam-structure within the web, said foam-structure being formed of aplurality of members, which members are separated from each other bychannels that are permeable to resin, wherein the members have anaverage diameter—as defined by the diameter of the enveloping circle, inthe plane of the material—of less than 1.5 mm, preferably of 0.2-1 mm,and wherein the channels have an average diameter of less than 0.75 mm,preferably of 0.3-0.5 mm.

Such a core material has been found very suitable for improving surfacequality and/or improving the visual appreciation, preferably withrespect to reducing the print-through effect, in laminate. More inparticular, it has been found that such a core material is very suitablefor providing a shaped article having an orange peel value of less than30.

For practical reasons the average diameter of the members will typicallybe at least 0.5 mm.

The members and channels may be distributed in a more or less regularway, e.g. with a pattern repeat of less than 1 cm, more in particular ofless than 0.5 cm, or in an irregular way as defined below.

For the other parameters, in particular permeability, nature of thematerials of which the core material is made, shape of the members, freevolume of the web, the conditions as described below apply.

The present invention further relates to a core material, in particulara core material suitable for use in closed mould systems, spray-upapplications and/or hand lay-up applications, said core material ingeneral being drapable, and preferably having a compression-resistanceof more than 30% at 1 bar pressure, wherein the core material is basedon at least one fibrous web containing a foam-structure within the web,said foam-structure being formed of a plurality of members, wherein themembers are irregularly distributed within or upon the web, wherein themembers are separated from each other by channels which channels arepermeable to a resin.

Such a core material has been found very suitable for improving surfacequality and/or improving the visual appreciation, preferably withrespect to reducing the print-through effect, in laminate. More inparticular, it has been found that such a core material is very suitablefor providing a shaped article having an orange peel value of less than30.

For the other parameters, in particular permeability, nature of thematerials of which the core material is made, shape of the members, freevolume of the web, the conditions as described below apply.

It has been found that a core material according to the inventionmaintains a very good drapability, compression resistance and a suitablepermeability, even in case of an irregular distribution of members,whilst the surface quality, in particular with respect to the visualappreciation is improved in comparison to known materials, e.g. a corematerial provided with a regular pattern of hexagons, such as Soric®,wherein a print-through effect, at the areas between the members (i.c.hexagons) may be clearly visible, after impregnation with the resin anddrying. It has been found that such a print through does not occur or atleast to a lesser extent with a core material according to the presentinvention.

Further it has been found that a core material according to the presentinvention has an improved drapability and/or permeability in comparisonto commercially available core materials.

FIGS. 1 and 2 show examples of how the members (the light areas) can bedistributed over the core material. The dark areas represent thechannels. These channels provide by and large the permeability in a corematerial represented by FIG. 1 or 2.

FIG. 3 shows an schematic example of a core material containing aplurality of differently shaped members FIG. 4 a shows a photograph of acommercially available core material (Soric®, with 6 mm hexagons) usedin the comparison example.

FIG. 4 b shows a photograph of a core material according to theinvention used in Example 1.

FIG. 4 c shows another core material according to the invention.

FIG. 5 shows two photographs comparing a composite comprising a corematerial according to the invention and a composite comprising a Soric®core material (each photographs shows the same two composites, butphotographs were taken from a different angle). The material accordingto the invention clearly has a more uniform visual appearance. This isfurther accentuated by the light bar in the central section of theplate, which is the reflection of the light of a common fluorescenttube. The distorted shape of the reflection in the prior art materialcompared to the material according to the invention is a strikingillustration of the visual improvement, due to use of the new corematerial.

FIG. 6 is a composite according to the invention, wherein a corematerial was used with a permeability of about 1.5×10⁻⁹ m². There is noprint-through effect visible at all, making this composite very highlyvisually appreciated.

FIG. 7 shows the diffraction pattern obtained by a D-Sight system of acomposite based upon Soric® core material formed of 6 mm hexagons. Theinserted rectangle in the lower middle section of the figure shows arepresentation at higher resolution.

FIG. 8 shows the diffraction pattern obtained by a D-SIGHT system of acomposite based upon Soric® core material formed of 3.5 mm hexagons.

FIG. 9 shows the diffraction pattern obtained by a D-SIGHT system of acomposite based upon a core material according to the invention (thecore material having a pattern as shown in FIG. 1, 1:1 scale).

An irregular distribution as used herein can be defined by its patternrepeat. The pattern repeat can be represented by the length (whendetermined in one direction) or area forming a pattern that is repeatedin a different part of the material. Good results have been achievedwith an irregular distribution, characterised in that in at least in onedirection of the x-direction and the y-direction and preferably in boththe x-direction and the y-direction, the pattern repeat of the patternof members is at least 0.5 cm, preferably at least 1 cm. The upper limitis not critical. For practical reasons, the upper limit may bedetermined based upon the technique by which the core material is made.For example, screen-printing is a very suitable technique to make a corematerial. The upper limit of the pattern repeat, will then in general bydetermined by the circumference and/or width of the screen. For example,screens with a circumference of up to about 92.5 cm are commonly used.In practice the upper limit will generally be about 140 cm.

It is also possible to determine the pattern repeat based upon thenumber of members forming a repeating pattern. Good results have beenachieved with an irregular distribution, wherein in at least in onedirection of the x-direction and the y-direction no repetition occurs inany pattern formed by at least 10 adjacent members and preferably in anypattern formed by at least 25 adjacent members. More preferably norepetition is visible in any members pattern in an area formed by atleast 100 adjacent members.

The irregular distribution is preferably mainly a random distribution,i.e. a distribution wherein no repetition of pattern of the membersoccurs within the plane of the core material. A core material with arandom distribution has been found to be very suitable for manufacturinga shaped article with a very high surface quality, in particular withrespect to the visual appreciation.

The orange peel value or D-Sight index as defined herein is measurableby the D-Sight technique (e.g. on a D-SIGHT system, supplied byDiffracto Ltd., Canada), see also above. Suitable conditions arespecified in the Examples, wherein the core material provided with afibrous glass fleece (such as chopped strand mat, CSM 450 g/m² suppliedby Owens-corning) at both sides and a Gelcoat™ (supplied by De IJssel,the Netherlands) surface finish. For comparative reasons it is notedthat a full glass material consisting of three layers of the fibrousglass fleece gives rise to an orange peel value of about 25 in thismethodology and use of a conventional core material (Soric®) a value ofabout 55.

Depending upon its intended use, in particular with respect to the needof a resin to be able to penetrate into the core material within aparticular time-span, the permeability for resin of a core materialaccording to the invention can be chosen in a wide range. Particularlygood results have been obtained with a core material having apermeability in the plane of the material for resin of at least 1×10⁻⁹m². In such a material the flow properties of the resin have been foundto be very satisfactory. For even better flow properties, thepermeability is preferably at least 1.5×10⁻⁹ m², more preferably atleast more than 5×10⁻⁹ m².

The permeability is largely provided by the channels, formed by theareas containing no members. The permeability (k) is defined hereinaccording to the law of Darcy for steady flow as${q = {\frac{k.A}{\eta} \cdot \frac{\Delta\quad p}{\Delta\quad x}}},$wherein q is the resin flow in m³/s, A is the total surface of the crosssection through which the resin flows in m², η is the viscosity of theresin in Ns/m², Δp is the pressure difference in N/m² and Δx is thedistance over which the pressure difference exists and the resin flowsin m. The permeability is defined in the plane of the material, that isnot perpendicular to the material, but parallel to the upper and lowersurface thereof.

The drapability is defined herein as the ability of the core material toconform to a contoured surface, in particular a mould. In particular acore material as defined herein is drapable, if it can be bent around acorner with a radius of 10 mm or less, without substantial irreversibledeformation of the core material. This allows the material to be drapedin a good way in the mould, thus enabling the production of smoothlyshaped products.

Although the above defined drapability is in general sufficient for usein close systems, it is an advantage that the present invention providesa core material with a much better drapability, such as a drapabilitythat allows bending around a corner with a radius of only 5 mm or less.

The compression resistance is defined herein as the ability to resist aforce that tends to crush or buckle. It is measured by determining theheight of the material before applying a pressure and during applying 1bar pressure perpendicular to the plane of the material. The compressionresistance is calculated as 100%×(height of the material at 1 barpressure)/height of the material at no pressure).

The compression resistance may be chosen in a wide range, depending uponthe type of application and the desired properties. Good results haveinter alia been achieved with a core material having a compressionresistance of at least 40% at 1 bar pressure. Especially in case thecore material should be suitable for a closed mould system, it is highlypreferred that the compression resistance is at least 60% at 1 barpressure, even more preferably about 70% or more at 1 bar pressure. Sucha resistance has been found highly advantageous because of a very lowtendency of the channels being pressed together, and thus compromisingthe entry of resin into the channels when being processed in a closedmould. Accordingly, a core material having a compression-resistance ofrespectively more than 75%, at least 80%, at least 90% or at least 95%at 1 bar pressure is highly preferred.

Nonetheless, under some circumstances one may opt for a core materialhaving a relatively low compression resistance, e.g. of about 50% orless.

In particular in case of a core material suitable for a hand lay up or aspray up system, a relatively low compression-resistance in principlesuffices, in particular a compression-resistance of 30% at 1 bar ormore.

The present invention combines the careful balancing of the propertiesof the various components, fibres, binder, foam structure and the liketo obtain a suitable balance between properties such ascompression-resistance, drapability and permeability in the corematerial on the one hand and to obtain a high surface quality in ashaped article formed with such a core material on the other hand.Suitable conditions can be determined by the skilled professional byroutine considerations and upon the information disclosed herein and inthe cited references

If a good surface quality is important but there is also a desire tolimit the use of resin and/or the weight of the final composite one maychoose to use a material for the members with relatively light material,e.g. a micro-sphere foam structure; a material with relatively largemembers, e.g. in the range of 1-3 mm; a material with relatively narrowchannels between the members, e.g. of less than 1 mm; and/or arelatively low free volume, e.g. in the range of 40-60 vol %.

If the surface quality is of the utmost importance and savings in weightor cost are of lesser importance, one may to choose a core material withrelatively small members, e.g. in the range of 0.5-2 mm (in case of acore material wherein the pattern is not irregular: 0.5-1.5 mm), a highdegree of irregularity of the member pattern and/or a resin with a lowtendency to shrink after curing, e.g. an epoxy resin.

If drapability and surface quality should be relatively high, one maychoose to use relatively wide channels, e.g. with an average diameter of0.5-2 mm (in case of a core material wherein the pattern is notirregular: 0.5-0.75 mm), in combination with relatively small members,e.g. with an average diameter of less than 1 mm, a high degree ofirregularity and/or a relatively flexible fibre material, e.g comprisingpolyester fibres and acrylate binder.

The members form ‘isles’ within or upon the web, which members are atleast largely surrounded by channels, through which channels resin canflow. The channels are largely free of web material or fibres, althoughsome fibre material may be present to provide sufficient consistency ofthe core material. As a rule the material content in the channels shouldbe low enough to allow a sufficient permeability to allow sufficientpenetration of resin, preferably it should allow a permeability of atleast 1×10⁻⁹ m².

The members are typically made of a closed cell foam structure, e.g.from a material that is usable as a binder material as disclosed herein.The members can also comprise micro-spheres or being formed thereof.These micro-spheres will be discussed below.

The members largely contribute to the compression resistance of the corematerial and are in general substantially impenetrable to the resin. Themembers in any case have a permeability of substantially less than1×10⁻⁹ m².

The members can have any shape. Good results have been achieved with acore-material wherein at least the majority of the members are selectedfrom the group consisting of members with circular, ellipsoidal andpolygonal cross-sections parallel to the plane of the material. Ofcourse combinations thereof may be employed. Preferred members withpolygonal cross-sections are members with triangular, tetragonal,pentagonal, hexagonal, heptagonal or octagonal cross-sections.

The irregular distribution may be obtained by using more or lessuniformly shaped members, with the same or different dimensions. Goodresults have for example been achieved with a core material wherein atleast the majority and preferably substantially all members have acircular or ellipsoidal cross-section parallel to the plane of thematerial.

The irregular distribution may be obtained by using a variety ofdifferently shaped members. Good results have been achieved with a corematerial wherein at least the majority and preferably substantially allmembers have a polygonal cross-section parallel to the plane of thematerial. The differently shaped members in such a core-material arepreferably selected from the group of triangles, tetragons, pentagonsand hexagons.

It has been found that particularly good results with respect to surfacequality are obtained with a core material having an irregular patternwherein at least the majority of the members and preferablysubstantially all members have a diameter, as defined by the diameter ofthe enveloping circle, in the plane of the material of less than 3 mm.Preferably, at least the majority of members and more preferablysubstantially all members have a diameter in the plane of the materialof less than 2.5 mm. Very good results have been obtained with a corematerial wherein at least the majority of the members have a diameter ofless than 1.5 mm.

The lower limit of the diameter of the members is not particularlycritical. For typical applications, at least the majority of the memberswill have a minimum diameter of at least about 0.2 mm. For practicalreasons the diameter will generally be at least about 0.5 mm. Factors,other than the surface quality, to which the diameter of the members maybe relevant is the extent to which one wishes to restrict the use ofresin in a closed mould system.

Preferably at least the majority of the channels between members have anaverage diameter of less than 2 mm (in case of an irregular pattern),more preferably of less than 1 mm (in case of an irregular pattern),even more preferably of less than 0.5 mm. The lower limit of thechannels is not particularly critical, as long as the permeabilityremains high enough, as defined herein. A suitable lower limit canroutinely be determined by the skilled professional, depending upon theresin and moulding conditions. Typically the majority of the channelswill have a minimum average diameter of at least 0.3 mm. Advantages ofusing a relatively high diameter, e.g. 0.5 to 2 mm (0.5-0.75 in case thecore material does not have an irregular pattern) may be a fast flow ofresin through the material and a relatively high drapability. Advantagesof a relatively low diameter, e.g. in the range of 0.3 to 0.5 mm mayinclude relatively low resin uptake and a higher surface quality.

The thickness of the core material can be varied within wide ranges,e.g. between 1 and 4 mm, preferably between 1.5 and 3 mm, althoughthicker or thinner core materials can be made in accordance with theinvention. The members usually at least extent to the majority of thethickness of the material.

Preferably the fibrous web containing a foam structure has a free volumeof less than 80 vol. %, more preferably of 50-70% by volume. In thisrespect the free volume is understood to mean the volume of the materialthat can be accessed by resin. The remainder of the volume will beformed by the members (and some fibres).

A preferred web comprises at least 20 wt. % of fibres, up to 80 wt. % ofbinder material, which is optionally foamed. The closed cell foamstructure forming the members can be prepared from (optionallyexpandable) micro-spheres which are introduced into the web using anoptionally foamed binder material.

Good results have been obtained with a core material containingmicro-spheres having an activation temperature of at least 120° C.,wherein the free volume in the web is at most 80 vol. %. The web may bemechanically, physically or chemically bonded.

Much preferred is a core material comprising at least 30 wt. % offibres, up to 70 wt. % binder material, optionally also containingexpandable micro-spheres. In practice the amount of expandablemicro-spheres will generally be less than 15 wt. %, preferably 1-10 wt %based upon the total weight of the core material.

Preferably the micro-spheres are expandable and more preferably theyhave an activation temperature of at least 120° C.

Very good results have been achieved with a core material whereinexpanded thermoplastic micro-spheres, e.g. of a thermoplastic polymerbased on an alkylmethacrylate, such as methyl methacrylate, acetonitril(such as polyacetonitril (PAN)), vinilydene chloride or a combinationthereof, are present in the web, said micro-spheres having an initialexpansion temperature below the curing temperature of the binder.Micro-spheres are commercially available, e.g. Expancel™ by AKZO-NOBEL.

The core material of the invention may be prepared using techniquesknown for producing the prior art core materials for the manualproduction of fibre reinforced plastic materials. The preparation mayfor example be based upon the methodology as described in EP 1 010 793.Preferably the material is made by rotary screen printing.

In a preferred method to produce a core material, expandablemicro-spheres are introduced into a fibrous web, using a bindermaterial, followed by expanding the micro-spheres and curing the binder.In a much preferred method the micro-spheres start to expand at atemperature below the curing temperature of the binder material.

The core material may suitably be prepared in a method wherein anon-woven is printed with a foam or an unfoamed binder, also containingexpanded micro-spheres, such as polymeric, glass or ceramicmicro-spheres.

In the case of use of expandable micro-spheres, it is preferred to usethe following process. First a dispersion of expandable micro-spheres ina binder material is prepared, which dispersion is optionally foamed.The initial expansion temperature of the micro-spheres is preferablybelow the curing temperature of the binder material. Subsequently, thenon-woven, having a thickness less than the required final thickness, isscreen printed with the dispersion. Following this, the material isdried and heated to the expansion temperature of the micro-spheres. Uponexpansion the temperature is further raised with the result that thebinder material cures and sets the micro-spheres in the web. In this waya core material according to the invention can be prepared.

The initial expansion temperature of the micro-spheres is preferablybetween 120 and 190° C. The curing temperature of the binder ispreferably above 170° C.

The fibrous web to be used according to the invention will usually be anon-woven, which may be reinforced, based on conventional fibres. Themanufacture of suitable non-wovens has for instance been described byDr. H. Jörder, “Textilien auf Vliesbasis” (D. V. R. Fachbuch, P. KepperVerlag). It is also possible to use a combination of a non-woven fibrousweb with a reinforcing fabric, one within or on top of the other.

The fibres of the web are preferably selected from the group of naturalfibres, glass fibres, metal fibres, ceramic fibres or synthetic fibres,such as acrylic, polyethylene, polypropylene, polyester, polyamide(aramide), carbon or polypropylene fibres and combinations thereof. Morepreferrably the fibres are selected from the group of glass fibres,polyester fibres, polyester-polyethylene bicomponent fibres andcombinations thereof. Very good results have been achieved withpolyester fibres. Polyester fibres have been found to have very goodadherence with the resin and tend to have a favourably low moisturecontent.

According to a very convenient method, the non-woven is based on acombination of polyester fibres and polyethylene-polyester bicomponentfibres (or other low temperature melting fibres or powders). These typesof webs have been thermally bonded by the bicomponent fibres. By heatingthe web to the initial expansion temperature of the micro-spheres, whichis above the melting point of the polyethylene bond, the web becomesloose and will expand easily. After expansion, and curing the finalmaterial again has its good bond, resulting in the advantageouscombination of properties of the invention. At the same time the web isvery easy to handle at the initial stages of the process, thanks to thethermal bonding.

The micro-spheres that may be provided in a fibrous web according to theinvention preferably at least consist of a thermoplastic synthetic resinmaterial that is solid at room temperature. Examples of suitable resinsinclude polystyrene, styrene copolymers, polyvinyl chloride, vinylchloride copolymers, vinylidene chloride copolymers and so forth.

In expandable micro-spheres, usually a blowing agent has beenincorporated. The presence of this blowing agent is responsible for anexpansion of the micro-spheres when a fibrous web, comprising themicro-spheres, is cured. Thus, the micro-spheres are pressed into thefibrous web in unexpanded form, for example by means of a paste, such asa foam paste. The blowing agent may be a chemical or physical blowingagent, such as azodicarbonamide, isobutane, isopentane, pentane, freon,iso-octane etcetera.

The micro-spheres advantageously have a diameter of 4-20 μm inunexpanded state, and a diameter of preferably 10-100 μm in expandedstate. After expansion of the micro-spheres, the amount thereof in theweb is in general 10 to 60 vol. %. This amount depends on the amount ofmicro-spheres used and the degree of expansion thereof.

Suitable binders in this regard are for instance lower alkyl acrylatepolymer, styrene-butadiene rubber, acrylonitrile polymer, polyurethane,epoxy resins, polyvinyl chloride, polyvinylidene chloride, andcopolymers of vinylidene chloride with other monomers, polyvinylacetate, partially hydrolyzed polyvinyl acetate, polyvinyl alcohol,polyvinyl pyrrolidone, polyester resins, and so forth. Optionally thesebinders can be provided with acidic groups, for example by carboxylatingthe binders. A suitable carboxylating agent is, for example, maleicanhydride. In addition, the binder, paste-like composition optionallycontains water, surfactants, foam stabilizers, fillers and orthickeners, as has been described in EP-A-0 190 788.

The present invention further relates to a laminate at least consistingof a core material according to the invention, laminated with at leastone fibrous fleece. The laminate may be formed in any way, andpreferably by stitching or gluing the at least one fleece to one or bothsides of the core material. Suitable methods of forming the laminate areknown in the art.

An advantage of providing a laminate is the ease of use. A laminateallows easy placement of the combination of core material and fleeces inone step. Thus the manufacturer of a composite does not have to stapledifferent layers (e.g. respectively bottom fleece, core material, topfleece) into the mould in separate steps.

In principle any fibrous fleece suitable for preparing a composite canbe used. Preferred fibrous fleeces include glass fibre fleeces, carbonfibre fleeces, polyaramide fibre fleeces and hybrids thereof, e.g.glass-carbon fibre fleeces, glass-polyaramide fibre fleeces orcarbon-polyaramide fibre fleeces.

A core material according to the invention has inter alia been foundvery suitable to make thin laminates, whilst obtaining a surface with ahighly desirable smooth appearance. For example a laminate according tothe invention may very suitably have a total thickness of 2 to 10 mm,preferably of 3 to 6 mm. Good results have inter alia been realised witha laminate of a core material with a thickness of 1-2 mm covered at bothsides with a fleece, preferably a glass fleece, with a thickness ofapproximately 0.4-0.8 mm, e.g. a glass fleece of approx. 225-600 g/m²,typically about 450 g/m². Thus a laminate can be obtained with athickness of about 2-3 mm, which laminate has been found to have a verygood surface quality after being cured with a resin, in particular withan epoxy resin.

The invention also encompasses a method for manufacturing a shapedarticle, wherein a fibrous web as described hereinabove is impregnatedwith a liquid resin and a hardener there for.

Suitable liquid resins for impregnating a fibrous web according to theinvention are any synthetic plastic materials that can be applied inliquid form and be cured. Examples are polyester resins, phenylesterresins, polyurethane resins, phenol resins, melamine formaldehyde resinsand epoxy resins. Given the specifications of a shaped article to bemanufactured, a skilled artisan will be able to suitably select anappropriate resin.

Suitable hardeners for use in a method according to the invention areany hardeners which can be used to cure the chosen liquid resin. Thesesystems are known to the skilled person. It belongs to the standardknowledge of the person skilled in the art to be able to combine resinand hardener so as to obtain optimum results.

The present invention further relates to a shaped article based upon acore material according to the invention, in particular a shaped articleobtainable by a method according to the invention wherein a corematerial according to the invention is impregnated with a resin andcured. In particular, the present invention relates to such a shapedarticle having a orange peel value or D-Sight index of less than 30,preferably less than 25, more preferably of 10-20, as determined by theD-Sight technique (e.g. on a D-SIGHT system, supplied by Diffracto Ltd.,Canada), using the conditions as specified in the Examples.

The invention will now be elucidated by the following, non-restrictiveexamples.

COMPARISON EXAMPLE

Soric® core material (Lantor, Veenendaal, the Netherlands) as shown inFIG. 4 a with hexagon members having a diameter of approximately 6 mm, apermeability of 5×10⁻⁹ m² was sandwiched between two layers of fibrousglass fleece (chopped strand mat) (material weight: 450 g/m²; suppliedby Owens-Corning). This laminate was impregnated with polyester resin(Synolite 6811-N-1, DSM resins) in a closed mould system by vacuuminjection at a pressure of 0.2 bara.

To one side of the mould a surface finish (black Gelcoat™ yt 701; fromDe IJssel) of about 0.5 mm was applied.

FIG. 5 shows the appearance of the composite (marked as “prior corematerial”). The orange peel value/D-sight index of the surface wasdetermined using a D-SIGHT system with the following parameter settings:Camera angle: 30°, Camera height: 570 Application point: “red”lineFragment size: Top left corner: x = 115, y = 271 Bottom right corner: x= 385, y = 386 WDI 28 × 24 Block size 10 × 5

The orange peel was found to be 55.3. The graph representing thecomposite as observed via the D-SIGHT system is shown in FIG. 7.

A second comparison example was performed in the same way, but with aSoric® core material having hexagons with a diameter of about 3.5 mm anda permeability of about 5×10⁻⁹ m². The orange peel was found to be 33.7.A graph representing the composite as observed via the D-SIGHT system isshown in FIG. 8.

A third comparison example was performed in the same way, but now with afull glass laminate (three layers of the same glass fleece). The orangepeel was found to be 24.9 (not shown in Figures).

Manufacture of the Core Material

A web was prepared consisting of 80 wt % polyester fibres and 20 wt %binder (acrylate).

A binder-microsphere mixture was made by mixing 5 kg of expandiblemicrospheres (Expancel™, AKZO-NOBEL into 95 kg of acrylate dispersion.The solids content was about 52 wt %.

The binder-microsphere mixture was applied to the web by rotary screenprinting, wherein the mixture was pressed into the web. The screencontained round irregularly distributed holes with an approximatediameter of about 0.6 mm, in a density of about 40 holes per cm².

After printing the web was dried at about 110° C. and subsequentlyexpanded to a thickness of about 2 mm at a temperature of 200° C.Simultaneously the web was cured.

Example 1

A core material with an irregular pattern as shown in FIG. 1 wherein themembers had an average diameter of approximately 1 mm, with apermeability of about 1.5×10⁻⁹ m² was provided with the glass mats, asindicated above. The laminate was impregnated according to the sameprocedure as described in the comparison example.

FIG. 5 shows the appearance of the composite (marked as “improved corematerial”).

FIG. 5 clearly demonstrates that the appearance of the core materialaccording to the invention yields in a composite of which the surfacehas a much smoother appearance, resulting in a higher visualappreciation.

A graph representing the composite as observed via the D-SIGHT system isshown in FIG. 9. The orange peel was found to be 20.6.

Example 2

FIG. 6 shows another composite according to the invention, made with acore material as shown in FIG. 4 c, which also shows a very uniformappearance.

1. A core material, suitable for use in a closed mould system, spray upapplication and/or hand lay up application, said core material beingdrapable, which core material is based on at least one fibrous webcontaining a foam-structure within the web, said foam-structure beingformed of a plurality of members, which members are separated from eachother by channels that are permeable to resin, wherein the members havean average diameter—as defined by the diameter of the enveloping circle,in the plane of the material—of less than 1.5 mm and wherein thechannels have an average diameter of less than 0.75 mm.
 2. A corematerial suitable for use in a closed mould system, spray up applicationand/or hand lay up application, said core material being drapable, whichcore material is based on at least one fibrous web containing afoam-structure within the web, said foam-structure being formed of aplurality of members, which members are irregularly distributed withinor upon the web, which members are separated from each other bychannels, which channels are permeable to resin.
 3. A core materialaccording to claim 2, wherein at least the majority of the members havea diameter, as determined by the enveloping circle surrounding themember, in the plane of the material of less than 3 min, preferably ofless than 2.5 mm.
 4. A core material according to claim 2, wherein atleast the majority of the channels have an average diameter of less than1 mm.
 5. A core material according to claim 2, wherein the channels havean average diameter of 0.3-0.5 mm.
 6. A core material according to claim2, wherein the members have an average diameter of 0.2-1 min.
 7. A corematerial according to claim 2, wherein the permeability in the plane ofthe material for resin is at least 1×10⁻⁹ m².
 8. A core materialaccording to claim 2, wherein the members are randomly distributedwithin or upon the web.
 9. A core material according to claim 2, whereinthe core material contains a plurality of differently shaped members.10. A core material according to claim 2, wherein the free volume of theweb is 40-80% by volume, preferably 60 to 70% by volume.
 11. A corematerial according to claim 2, wherein the cross-sections parallel tothe plane of the material of at least the majority of the members areselected from the group consisting of circular, ellipsoidal andpolygonal cross-sections.
 12. A core material according to claim 2wherein at least part of the members contain micro-spheres.
 13. A corematerial according to claim 2, wherein the fibres of the web areselected from the group consisting natural fibres, glass fibres, metalfibres, ceramic fibres, synthetic fibres and combinations thereof,
 14. Acore material according to claim 2, having a compression resistance at 1bar of at least 30%, preferably at least 60%, more preferably at least70%.
 15. A laminate at least consisting of a core material according toclaim 2, laminated with at least one fibrous fleece.
 16. A laminateaccording to claim 15, wherein the laminate has a total thickness of 1to 10 mm, preferably of 2 to 5 mm.
 17. A laminate according to claim 15,wherein at least one fibrous fleece is selected from the groupconsisting of at least one type of fibre selected from the groupconsisting of glass fibres, carbon fibres and polyaramide fibres.
 18. Alaminate according to claim 15 wherein the at least one fibrous fleeceis glued or stitched to the core material.
 19. A process for preparing ashaped article, said process comprising placing; a core materialaccording to claim 1 optionally in combination with one or more othernon-woven fleeces, or a laminate in a closed mould, introducing a liquidresin into the mould and curing the resin to produce the article. 20.Process according to claim 19, wherein the resin is a polyester resin, aplienylester resin, an epoxy resin, a polyurethane resin, a melamineformaldehyde resin or a phenol resin.
 21. A shaped article obtainable bya process according to claim
 19. 22. A shaped article, based upon a corematerial according to claim 14 or a laminate.
 23. A shaped articleaccording to claim 22, having a diffraction index as represented by A;orange peel value of less than 30, preferably of less than 25, morepreferably of 10-20.
 24. Process for producing a core material accordingto claim 1, said process comprising introducing a foamed or foamgenerating material into a fibrous web using at least one bindermaterial and setting a foam in the web by curing the binder material.25. Process according to claim 24, wherein the foamed or foam generatingmaterial is introduced into the web by rotary screen printing.